Automated mechanical transmissions, such as single input shaft transmissions or dual clutch transmissions, may be equipped with an alternative power source to provide a hybrid drive system. The alternative power source may be an electric machine that is configured to operate as a motor to provide torque and operate as a generator to convert torque to electrical power to charge a battery.
Prior art systems may connect the alternate power source to the transmission in various ways. Often, the electric machine is placed between the master starting clutch and the input shaft to the gearbox of the transmission, and may be referred to as a front motor configuration. This configuration may be used with both medium-duty transmission applications typically having five or six gear ratios and heavy-duty transmission applications typically having ten or more gear ratios.
The front motor configuration has various advantages. The front motor configuration is a parallel architecture in that the engine and electric machine power travel the same power or torque paths to offer redundant operation characteristics. For example, if the electric machine system loses functionality, the vehicle may still operate using the engine and transmission. The front motor configuration is simple, efficient, and modular. The electric machine may use all of the transmission speed or gear ratios for torque multiplication. Also, the electric machine operates in a similar speed range as the engine, which may correspond with an efficient speed range of the electric machine for a large portion of the duty cycle.
However, the transmission may also be limited by a front motor configuration. For a single input, automated mechanical transmission, the transmission interrupts torque delivered through the vehicle driveline to the wheels during all gearshifts, and this interrupts power during acceleration or deceleration. For example, power delivered by an engine and an electric machine is brought to zero for a period of time (i.e., one to two seconds) to allow the transmission to shift by disengaging the current gear and engaging the new gear during an acceleration event. Similarly, the electric machine torque is brought to zero to facilitate downshifts during vehicle braking and deceleration where the electric machine is used as a generator. These torque interrupts during a shift event adversely affect vehicle performance, drivability, and user expectations for a transmission with a front motor configuration.
With a front motor configuration, the combined power and torque of the engine and the electric machine flow through the input shaft of the transmission. Therefore, the sum of the maximum torque of the engine and electric machine cannot exceed the torque rating of the transmission. Also, the operating speed range of the electric machine is largely dictated by the engine speed range, and therefore needs to be a relatively large, slow-turning electric machine unless a gear reduction is placed between the electric machine and the transmission input shaft that allows a smaller, less-expensive, higher-speed electric machine to be used.
Packaging may also be a concern, as the transmission assembly can become long with the front motor configuration. Also the transmission may need to be designed or assembled with the hybrid drive system incorporated from the outset, where the transmission and electric machine system are not modular such that the transmission may be used alone or in combination with the electric machine based on transmission or vehicle requirements.
Other automated mechanical transmissions and dual clutch transmissions use an electric machine positioned elsewhere in the transmission; however, the power or torque flow path of the electric machine typically flows through an input shaft of the transmission or relatively early in the transmission power path causing torques approaching the torque limit of the transmission, leading to the same limitations as the front motor configuration.